The present invention relates to a digital image processing method suitable for halftone image representation based on an improved dither method in which tone or gradation is represented by changing the ratio of recording dots to non-recording dots in a unit area, the digital image processing method producing a halftone image with a high gradation and high resolution in which one dot is represented with a plurality of tone levels.
A digital image processing apparatus is designed to process image data in a digital form. For example, it is known that digital image data can be represented by the dot matrix method. In the dot matrix method, each dot is represented by only a few tone (gradation) levels. However, in order to record a high-quality image relating to photographs or pictures on a recording medium such as paper, a dot should be represented by a plurality of tone levels for each of the basic colors such as yellow (Y), magenta (M), cyan (C) and black (Bk).
In order to enable the above mentioned multilevel image representation, it has been proposed that a dot area, which has a fixed area and is composed of a plurality of dots, be defined as a unit area for the tone production process, and the tone representation be made by changing the number of recording dots and the number of non-recording dots contained in the unit area. Examples of this type of image representation are a tone production method employing a density pattern, and a dither method.
However, the conventional multilevel image representation has a disadvantage in that as the number of tone levels increases, the resolution is deteriorated. An increase in the number of tone levels is suitable for images of photographs, but, on the other hand, it is unsuitable for character images and line images when it is desired to reproduce images with a high resolution. Therefore, it is preferable to represent character and line images by using a bi-level signal.
In the conventional dither method, which of various dither matrix patterns is used is based on whether the tone representation or the resolution should be mainly considered. When great importance is attached to the number of tone levels (tone representation), a dot concentration pattern is used. On the other hand, when great importance is attached to the resolution, a dot scattering pattern is used.
An improved image processing method based on the above mentioned dither method has been proposed in order to provide an image processing method for increasing both the gradation and the resolution of an output image (see Japanese Patent Publication "Kokoku" Nos. 64-1992 and 1-31753). In the improved image processing method, the tone representation is made by changing the ratio of recording dots to non-recording dots contained in a unit area. However, the improved image processing method has a disadvantage in that although the line density of an output image is increased to give the image a higher resolution, the gradation thereof is deteriorated.
Due to recent developments in image processing technology of a laser printer, a tone representation of each pixel of an image made by an increased number of tone levels (e.g. 256 levels) has been realized. An improved image processing based on the recent laser printer technology mentioned above has been proposed. In order to represent a dot by a plurality of tone levels, in the improved image processing, the laser pulse width which is proportional to the amount of electrostatic charge on a photosensitive medium for printing a dot is modulated in accordance with image data relating to the dot (see Japanese Patent Publication "Kokoku" No. 3-59622). However, the proposed method mentioned above has a disadvantage in that although the resolution is increased by increasing the line density of the output image, the gradation thereof is deteriorated.
In a conventional image forming process, the tone representation relating to one dot which is made by an increased number of tone levels is somewhat unstable when a gradation characteristic varies. For example, FIGS. 4A and 4B show two gradation characteristics indicating the relationship between the input image level and the output image gradation. The gradation characteristic may vary depending on the temperature and humidity. A printed image based on the gradation characteristic shown in FIG. 4A is different from a printed image based on the gradation characteristic shown in FIG. 4B.
In addition, in a conventional image forming process, the quality of the output image is determined depending on the size of a dot being formed. For example, when a dot having a relatively large size is formed, a relatively large amount of toner is used to record the dot, so that the recorded dot will be clear and have a regular shape. When a dot having a relatively small size is formed, only a small amount of toner is used to record the dot, so that the recorded dot will be faded or have an irregular shape.
In addition, in a conventional printing technology, the screen angle of a dot image (mesh pattern) for each of the basic colors is changed by selecting one of various dither matrix patterns in the dither processing prior to the printing of an output image. In order to use various dither matrix patterns in the dither processing for the basic colors, it is necessary to store all the dither matrix patterns in a storage memory, thereby making the capacity needed for the memory to store the matrix patterns large.